Through hardness measurement and metallographic inspection, it is confirmed that the fatigue fracture of the rear axle axle of the automobile is caused by improper heat treatment, which makes more ferrite appear in the structure of the axle axle, resulting in insufficient hardness and strength.
Chemical composition analysis and hardness determination
1. Chemical composition analysis (GB3077-881).
2. Hardness determination 40CrZB/T21004-89 "Technical Conditions of Automobile Half Axle" standard, the core hardness of the shaft after pre-tempering is 24^30HRC, after intermediate frequency quenching treatment, the surface hardness of the rod is 52HRC 3 hardness units. The actual test results show that the hardness of the half shaft is low.
Macro and micro inspection
1. Macroscopic inspection The fracture occurs at the spline, the fracture is chrysanthemum-shaped, the entire section is inverted conical, and the crack is first generated on the outside of the spline shaft. The core area, which is gray-black in color and has no metallic luster, is the final instantaneous fracture zone.
2 Crack analysis
(1) There are few inclusions in the fractured half-shaft matrix, and there are no inclusions on both sides of the crack, but there is oxide scale in the crack, so it does not have the characteristics of cracks caused by non-metallic inclusions;
(2) There is no decarburization on both sides of the crack, the lines are smooth, the tail is slender, and not round, and the non-quenched cracks caused by the defects of the raw material itself (white spots, looseness, peeling, and subcutaneous bubbles) are excluded;
(3) The crack depth exceeds the hardened layer, and the sorbite and troostite structure in the hardened layer are fine and uniform, and quenching cracks caused by improper quenching such as excessive quenching temperature are excluded.
3 Microstructure
According to relevant literature, the unhardened layer of 40Cr steel after quenching and tempering is troostite and sorbite structure, and ferrite is allowed to exist in the core.
Samples were taken from the fractured part of the spline shaft and observed under an optical microscope. The spline tooth structure was tempered sorbite and tempered troostite. The matrix structure of the semi-axial part is sorbite, on which there are reticular and needle-like ferrites precipitated along the grain boundary, and the black aggregates are troostite. It can be seen that the ferrite content gradually increases with the distance from the bottom of the spline to the center.
4 Discussion
According to the composition analysis, the chemical composition of the material used in the semi-shaft meets the composition requirements of 40Cr in the standard GB3077-88, and the steel is pure, so the factor of fracture caused by misuse of materials and poor steel can be excluded.
The hardness test results at the half shaft spline show that the hardness values from the core to the tooth are obviously low. Because the working environment of the half shaft is harsh and it is subjected to two-way alternating torsional stress, and the spline shaft is the force fulcrum, insufficient hardness may lead to the formation of a fatigue core at the angle of each groove, and at the same time The two sides at 45o expand in the oblique direction and merge at the center of the shaft, and finally form a star-shaped fracture.
From the microstructure point of view, the semi-axle contains more ferrite, which is precipitated in the form of nets and needles along the grain boundaries. Generally, automobile half shafts need to be quenched and tempered, that is, the half shafts are heated to Ac3+ (30～50℃), kept for a period of time, and then cooled at a rate greater than the critical cooling rate, so that the austenite is supercooled without touching the nose tip of the C curve. All transformed into martensite. If the cooling rate is less than the critical cooling rate, part of the austenite will transform into troostite and bainite, and ferrite will preferentially precipitate along the grain boundaries, resulting in a decrease in the hardness and strength of the steel. There are generally two reasons for this situation: one is improper selection of the cooling medium; the other is due to mass production, too much furnace loading, and poor cooling and heating cycles due to the accumulation of parts, which makes the C curve to the left, which is bound to be after quenching. Troostite and unmelted ferrite structure appear. The undissolved ferrite in this quenched structure cannot be removed by high temperature tempering. The purpose of tempering is mainly to eliminate internal stress caused by quenching and lattice distortion, reduce hardness, improve plasticity and toughness, and will not change the existing ferrite. The fine mesh ferrite appearing in the semi-axial microstructure is also the structure that exists after quenching. The difference is that massive ferrite is caused by low quenching temperature and insufficient holding time, and insufficient austenitization, while reticulated ferrite is due to the slower cooling rate during the cooling process, and ferrite has priority along the grain boundary. Because of precipitation.
In the requirements of quenched and tempered steel, it is not allowed to have more free ferrite in the tempered structure, especially the fine mesh free ferrite distributed along the grain boundary. It not only reduces the strength, but also directly affects the fatigue fracture. Because the damage of steel parts always starts from the free ferrite with lower strength, especially the steel parts that work under complex alternating stress, once there is free ferrite in the core, the ferrite is in cold working during work. In the hardened state, with the extension of working time, it will develop from hardening to embrittlement to a certain limit, and then brittle fracture. In addition, due to the difference in the strength and plastic deformation of ferrite and sorbite, steel parts undergo different plastic deformations when they are subjected to the same stress. In the two adjacent components, the grain boundary part produces greater stress. And deformation. Once this residual stress and deformation exceed the crack resistance of the steel, it will cause the grain boundary to crack, and when it continues to expand, it will become the main source of fatigue fracture. Therefore, the presence of more ferrite in the semi-axial structure is the root cause of the fracture.
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